linux-kernel - Re: [PATCH v4 1/1] PCI: dwc: Fix index 0 incorrectly being interpreted as a free ATU slot

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Message-ID: <20240517170650.GC1947919@rocinante>
Date: Sat, 18 May 2024 02:06:50 +0900
From: Krzysztof Wilczyński <kw@...ux.com>
To: Frank Li <Frank.Li@....com>
Cc: cassel@...nel.org, bhelgaas@...gle.com, gustavo.pimentel@...opsys.com,
	helgaas@...nel.org, imx@...ts.linux.dev, jdmason@...zu.us,
	jingoohan1@...il.com, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, lpieralisi@...nel.org, mani@...nel.org,
	robh@...nel.org
Subject: Re: [PATCH v4 1/1] PCI: dwc: Fix index 0 incorrectly being
 interpreted as a free ATU slot

Hello,

> When PERST# assert and deassert happens on the PERST# supported platforms,
> the both iATU0 and iATU6 will map inbound window to BAR0. DMA will access
> to the area that was previously allocated (iATU0) for BAR0, instead of the
> new area (iATU6) for BAR0.
> 
> Right now, we dodge the bullet because both iATU0 and iATU6 should
> currently translate inbound accesses to BAR0 to the same allocated memory
> area. However, having two separate inbound mappings for the same BAR is a
> disaster waiting to happen.
> 
> The mapping between PCI BAR and iATU inbound window are maintained in the
> dw_pcie_ep::bar_to_atu[] array. While allocating a new inbound iATU map for
> a BAR, dw_pcie_ep_inbound_atu() API will first check for the availability
> of the existing mapping in the array and if it is not found (i.e., value in
> the array indexed by the BAR is found to be 0), then it will allocate a new
> map value using find_first_zero_bit().
> 
> The issue here is, the existing logic failed to consider the fact that the
> map value '0' is a valid value for BAR0. Because, find_first_zero_bit()
> will return '0' as the map value for BAR0 (note that it returns the first
> zero bit position).
> 
> Due to this, when PERST# assert + deassert happens on the PERST# supported
> platforms, the inbound window allocation restarts from BAR0 and the
> existing logic to find the BAR mapping will return '6' for BAR0 instead of
> '0' due to the fact that it considers '0' as an invalid map value.
> 
> So fix this issue by always incrementing the map value before assigning to
> bar_to_atu[] array and then decrementing it while fetching. This will make
> sure that the map value '0' always represents the invalid mapping."

Applied to controller/dwc, thank you!

[1/1] PCI: dwc: Fix index 0 incorrectly being interpreted as a free ATU slot
      https://git.kernel.org/pci/pci/c/cd3c2f0fff46

	Krzysztof